RADIANT HEATING WITH INFRARED - Watlow

Temperature is the driving force in
radiant energy. The Stefan-Boltzman
equation is the heart of the formula
used to calculate the radiant energy
transferred from heater to product.
6
A surface at
1200˚F
radiates over
twice the
energy that it
radiates at
900˚F.
Radiant Heat Transfer Formula
When calculating how much heat energy can be put into a product using
radiant heaters, there are three important physical parameters to consider:
• Stefan-Boltzman Equation is used to calculate the amount of power
radiated by the heater.
• View Factor, usually determined graphically, describes what percentage of
the energy radiated by the heater actually hits the product.
• Emissivity of the product determines how much of the incident radiant
energy is actually absorbed by the product.
When combined into an equation these parameters are used to calculate the
net watts absorbed by a product being heated with radiant panels.
The Stefan-Boltzman Equation
All objects with a temperature above absolute zero radiate energy. The hotter
the object, the greater the amount of energy radiated in a given time period
(Power = Energy/Time). The Stefan-Boltzman equation calculates the amount
of power (watts) radiated by a blackbody surface at temperature T.
Watts Radiated = Constant x (absolute temperature) 4
Area
Watts = (0.1714 x 10 -8 BTU/Hrft 2 °R) (°F + 460) 4
ft 2
3.412 BTU/watt hr
From this equation, it can be seen that the watts radiated from the object
depends on the absolute temperature of the radiating surface to the fourth
power. This means that a small increase in the temperature will produce a
large increase in the radiated watts. Figure 6 shows the relationship between
radiated power and temperature.
Radiated Power vs. Temperature (Fig. 6)
W/in2 (Blackbody)
130
120
110
110
90
80
70
60
50
40
30
20
10
0
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Surface Temperature (˚F)